1. Field of the Invention
The present invention concerns temperature-stabilized microwave filters. It is more particularly concerned with resonant cavity filters in which tuning screws are used for final electrical adjustment of the cavities and temperature stabilization of the filter, in other words maintenance of the electrical response characteristics of the filter over a wide range of temperatures.
2. Description of the Prior Art
It is known to constitute a microwave filter by means of coupled resonant cavities. The resonant frequency in each cavity is determined by the dimensions of the cavity.
Precise adjustment of the electrical length of each resonant cavity is obtained by means of a tuning screw, of metal or preferably of an insulating material, inserted through a wall of the cavity and extending to a varying degree into the cavity so as to tune the cavity to the required frequency. These screws avoid the need for accurate machining of the cavity and final adjustment of the cavity dimensions, operations incompatible with mass production, and provide the final adjustment of the cavity resonant frequency.
Filters manufactured in the usual manner from brass or aluminum and tuned in the manner described hereinabove are nevertheless subject to frequency drift when the metal of which they are constituted expands or contracts with variations in the ambient temperature. This drift in the tuned frequency of each cavity is caused by mechanical variations in the length of the cavity and the larger dimension of the transverse cross-section of the cavity, increasing the cavity volume when the temperature increases and so reducing the tuned frequency.
In order to compensate for this tuning drift as a function of temperature, it is known to use tuning screws of a thermally stable material attached to the body of the filter in such a way that they move outwardly relative to the cavities when the filter body expands. When the tuning rod moves outwardly relative to the cavity, the cavity tuned frequency is increased.
In French Pat. No. 2 326 077, adjustment of the cavity tuned frequency and temperature compensation of the cavity tuning drift and filter passband drift are obtained using tuning screws with shanks made from a thermally stable material and threaded ends screwed into the body of the filter forming, for each screw and in each cavity, a tubular support delimiting a conduit longer than the wall of each cavity with which the shank projecting into the cavity does not come into contact. For example, the shank consists of a quartz cylinder bonded to a threaded end made from an alloy material known as Invar. Appropriate selection of the diameter and length of the quartz shank and its anchor point provides very good compensation of frequency drift with temperature.
While the operation of the filters obtained in this way is satisfactory, it is nevertheless the case that their cost is high. Also, mating without play of the thread of the screw end with the thread in the filter body is a difficult operation ruling out large-scale industrial manufacture of these filters.
An objective of the present invention is to propose a considerably simplified filter construction based on the same fundamental idea as the aforementioned disclosure, that is to say displacing the point of attachment of the tuning screw of the cavity concerned. The filter in accordance with the invention is made by mechanical machining at low cost.